Britain yesterday described as "unacceptable" the smuggling of weapons from Iran into Iraq after revealing that a consignment was intercepted at the border between the two countries.

While complaints have been made in the past, it is relatively rare to have concrete evidence of such smuggling.

The British embassy in Tehran raised the issue at a meeting with the Iranian foreign ministry. Officials relayed the government's concern and pressed Iran to acknowledge that there was a problem that should be dealt with.

Donald Rumsfeld, the US secretary of defence, warned Iran this week about the extent of smuggling. The US has been protesting for the past two years over alleged Iranian meddling in Iraq. Mr Rumsfeld told a Pentagon briefing that the smuggling was "a problem" for the Iraqi government. "It's a problem for the coalition forces. It's a problem for the international community, and ultimately, it's a problem for Iran," he said.

Disclosure of the smuggling came hours after four American soldiers were killed and six were wounded as a patrol was attacked near Baiji, 112 miles north of Baghdad, late on Tuesday. A bomb wrecked two Humvees and a bigger armoured vehicle.

Iran has repeatedly denied any involvement in the insurgency or party politics in Iraq.

A senior British official disclosed yesterday details of the incident two weeks ago when a group crossing from Iran was intercepted near Maysan, which is in the British controlled sector of Iraq. Iraqi security forces opened fire and the smugglers fled back to Iran leaving their cache of timers, detonators and other bomb-making equipment.

The British official said he did not know the identity of the group or those behind it but said it had the "fingerprints" of either Iran's Revolutionary Guard, controlled by the supreme leader Ayatollah Ali Khamenei, or the Lebanese based Hizbullah which Tehran backs. The incident came against a backdrop of tension between Iran and the west over allegations that Tehran is intent on securing a nuclear-weapons capability.

The US has had no diplomatic relationship with Iran since 1980 and has branded it part of the "axis of evil". But Britain usually opts for a less confrontational approach than the US. The British official said he thought such smuggling from Iran was infrequent and trivial compared with the weapons going into Iraq from Syria.

Bayan Jabr, Iraq's interior minister, also played down the incident, saying it "was very much exaggerated".

Ibrahim al-Jaafari, Iraq's prime minister who spent years in exile in Iran, dodged questions yesterday about the alleged use of Iranian weapons by insurgents.

Iran has a vested interest in maintaining a degree of instability in Iraq to ensure the US and Britain leave but it does not want anarchy threatening its own security. Events in Iraq are going in the direction Tehran would have wished with its Shia co-religionists dominant and an increased Islamisation in the British sector.

Iran can exert influence through the many prominent Iraqis who were exiled in Tehran and via the Badr brigades, the Iraqi Shia militia that was based in Iran.

The British claim the Badr brigades have been disbanded but although they have swapped their uniforms for Iraqi police or army gear many of the men retain their original allegiances.